Formation of a vicinal semiconductor-carbon alloy surface and a graphene layer thereupon

ABSTRACT

A surface of a single crystalline semiconductor-carbon alloy layer having a surface normal along or close to a major crystallographic direction is provided by mechanical means such as cutting and/or polishing. Such a surface has naturally formed irregular surface features. Small semiconductor islands are deposited on the surface of single crystalline semiconductor-carbon alloy layer. Another single crystalline semiconductor-carbon alloy structure may be placed on the small semiconductor islands, and the assembly of the two semiconductor-carbon alloy layers with the semiconductor islands therebetween is annealed. During the initial phase of the anneal, surface diffusion of the semiconductor material proceeds to form vicinal surfaces while graphitization is suppressed because the space between the two semiconductor-carbon alloy layers maintains a high vapor pressure of the semiconductor material. Once all semiconductor material is consumed, graphitization occurs in which graphene layers can be formed on the vicinal surfaces having atomic level surface flatness.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This disclosure was made with government support under Defense AdvancedResearch Project Agency (DARPA) CERA Contract No. FA8650-08-C-7838awarded by the U.S. Department of Defense. The government has certainrights in this disclosure.

BACKGROUND

The present disclosure relates to a method of forming a vicinalsemiconductor-carbon alloy crystalline surface.

Referring to FIG. 1, an annealed surface of a single crystalline siliconcarbide substrate is shown. The surface of the single crystallinesilicon carbide substrate is pitted because selective evaporation ofsilicon atoms during anneal at an elevated temperature of about 1,450°C. results in formation of pitted surfaces and graphitization on allexposed surfaces. The flat regions of the graphene layers formed on suchsurfaces do not laterally extend over a wide area due to the presence ofthe pits, and consequently, the graphene layer quality may not be ashigh as it could if it consisted of only flat regions. Since thegraphene layers on the surface follow the contours of the pitted surfaceof the single crystalline silicon carbide substrate, the orientations ofthe graphene layers vary. The randomness of the orientations of thegraphene layer on this type of surface combined with the surfacetopography makes it difficult to provide useful devices employing thefragmented graphene layers on this type of surface. Therefore, acontiguous sheet of a graphene layer having a clearly defined constantorientation over a large area would be desirable.

BRIEF SUMMARY

An embodiment of the present disclosure provides a method of forming acontiguous graphene layer having a constant orientation over a largearea of an unpitted smooth surface of a single crystallinesemiconductor-carbon alloy layer.

A surface of a single crystalline semiconductor-carbon alloy layerhaving a surface normal along or close to a major crystallographicdirection is provided by mechanical means such as cutting and/orpolishing. Such a surface has naturally formed irregular surfacefeatures. Small semiconductor islands are deposited on the surface ofsingle crystalline semiconductor-carbon alloy layer, for example, byannealing the single crystalline semiconductor-carbon alloy layer in anenvironment in which the semiconductor material of the singlecrystalline semiconductor-carbon alloy layer is externally providedeither as a gas or an influx of a beam. Another single crystallinesemiconductor-carbon alloy wafer, or a layer of a single crystallinesemiconductor-carbon alloy deposited on an appropriate support wafer,may be placed on the small semiconductor islands, and the assembly ofthe two semiconductor-carbon alloy layers with the semiconductor islandstherebetween is annealed. During the initial phase of the anneal,surface diffusion of the semiconductor-carbon alloy material proceeds toform vicinal surfaces while graphitization is suppressed because thespace between the two semiconductor-carbon alloy layers maintains a highvapor pressure of the semiconductor material. Once all the semiconductormaterial is consumed, graphitization occurs in which graphene layers canbe formed on the vicinal surfaces having atomic level surface flatness.

According to an aspect of the present disclosure, a method of forming avicinal surface on a single crystalline semiconductor-carbon alloy layeris provided. The method includes: forming semiconductor islands on asurface of a single crystalline semiconductor-carbon alloy layer; andannealing the single crystalline semiconductor-carbon alloy layer whilea substrate is placed on the semiconductor islands, wherein the surfaceof the single crystalline semiconductor-carbon alloy layer becomes avicinal surface during the annealing.

According to another aspect of the present disclosure, a method offorming a graphene layer is provided. The method includes: formingsemiconductor islands on a surface of a single crystallinesemiconductor-carbon alloy layer; annealing the single crystallinesemiconductor-carbon alloy layer while a substrate is placed on thesemiconductor islands, wherein the surface of the single crystallinesemiconductor-carbon alloy layer becomes a vicinal surface during theannealing; and continuing the annealing after formation of the vicinalsurface, wherein a graphene layer including at least one graphenemonolayer is formed at the vicinal surface as semiconductor atomsevaporate from the vicinal surface.

According to yet another aspect of the present disclosure, a structureis provided, which includes: a single crystalline semiconductor-carbonalloy layer having a vicinal surface; and a graphene layer including atleast one graphene monolayer and located at the vicinal surface.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an atomic force microscopy (AFM) image of a 20 μm×20 μm areaof a surface of a prior art single crystalline silicon carbide substratethat has been annealed at an elevated temperature about 1,450° C. in anargon ambient. A severe surface pitting occurred on the surface. Whilegraphene layers are present on the surface, the graphene layers followthe contour of the underlying silicon carbide substrate.

FIG. 2 is a schematic vertical cross-sectional view of an exemplarystructure prior to deposition of semiconductor islands according to anembodiment of the present disclosure.

FIG. 3 is a schematic vertical cross-sectional view of the exemplarystructure after deposition of semiconductor islands according to anembodiment of the present disclosure.

FIGS. 4A, 4B, 4C, and 4D are atomic force microscopy (AFM) images of asample of a single crystalline silicon carbide surface on which siliconislands are formed according to an embodiment of the present disclosure.

FIG. 5 is a schematic vertical cross-sectional view of the exemplarystructure after placement of a substrate according to an embodiment ofthe present disclosure.

FIG. 6 is a schematic vertical cross-sectional view of the exemplarystructure during an anneal of an assembly including the singlecrystalline semiconductor-carbon alloy layer and the substrate with thesemiconductor islands therebetween according to an embodiment of thepresent disclosure.

FIG. 7 is a schematic vertical cross-sectional view of an exemplarysubstrate having a vicinal surface according to an embodiment of thepresent disclosure.

FIG. 8 is a schematic vertical cross-sectional view of a first exemplarystructure after an anneal according to a first embodiment of the presentdisclosure.

FIG. 9 is a schematic vertical cross-sectional view of the firstexemplary structure after separation of the single crystallinesemiconductor-carbon alloy layer and the substrate according to thefirst embodiment of the present disclosure.

FIG. 10 is a schematic vertical cross-sectional view of a secondexemplary structure after formation of a graphene layer according to asecond embodiment of the present disclosure.

FIG. 11 is a schematic vertical cross-sectional view of the secondexemplary structure after separation of the single crystallinesemiconductor-carbon alloy layers and the substrate according to thesecond embodiment of the present disclosure.

FIGS. 12A, 12B, and 12C are atomic force microscopy (AFM) images ofvicinal surfaces including a graphene layer on silicon carbide substratesamples annealed according to the second embodiment of the presentdisclosure.

FIG. 13 is a graph showing the Raman spectroscopy data from a surface ofa sample of a vicinal surface including a graphene layer formed on asilicon carbide substrate sample annealed according to the secondembodiment of the present disclosure.

FIG. 14 is a vertical cross-sectional view of an exemplarygraphene-based field effect transistor according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to a method of forming agraphene layer on a carbon-containing semiconductor layer, and astructure obtained by the same, which are now described in detail withaccompanying figures. It is noted that like and corresponding elementsare referred to by like reference numerals. The drawings are not inscale. In drawings including a coordinate system, the x-axis is along ahorizontal direction within the plane of the drawing, the y-axis isalong a direction perpendicular to the plane of the drawing, and thez-axis is along a vertical direction within the plane of the drawing.

As used herein, a “vicinal” surface is a surface of a single crystallinematerial of which the surface normal deviates slightly from a majorcrystallographic orientation.

A crystallographic orientation is considered to a “major”crystallographic orientation if all

Miller indices are less than 7 in absolute value.

A surface normal is considered to deviate “slightly” from a majorcrystallographic orientation if the angle between the majorcrystallographic orientation and the surface normal is less than 2degrees.

As used herein, an “atomic bilayer” refers to a set of at least onelayer consisting of a layer of semiconductor atoms and a layer and alayer of carbon atoms that constitute the semiconductor-carbon alloymaterial of a single crystalline semiconductor-carbon alloy layer.

Referring to FIG. 2, an exemplary structure according to an embodimentof the present disclosure includes a single crystallinesemiconductor-carbon alloy layer 10. The single crystallinesemiconductor-carbon alloy layer 10 may constitute a stand-alonesubstrate, or may be located on a handle substrate (not shown) such thatthe substrate includes a stack of the single crystallinesemiconductor-carbon alloy layer 10 and the handle substrate. If ahandle substrate is present, the material of the handle substrate isselected such that the melting temperature of the material of the handlesubstrate is considerably greater than the temperature of an anneal tobe subsequently employed, and its vapor pressure is negligible at thetemperature of the anneal to avoid incorporation of its atoms into thesubstrate. For example, the handle substrate, if present, may be highpurity graphite or a refractory metal substrate.

The single crystalline semiconductor-carbon alloy layer 10 is a singlecrystalline material including at least one component semiconductormaterial and carbon. For example, the single crystallinesemiconductor-carbon alloy layer 10 may be a single crystalline siliconcarbide layer, a single crystalline germanium carbide layer, a singlecrystalline layer of a carbide of a silicon-germanium alloy, or anyother single crystalline alloy of at least one component semiconductormaterial and carbon. In case the single crystalline semiconductor-carbonalloy layer 10 is a single crystalline silicon carbide layer, the singlecrystalline silicon carbide layer may have a hexagonal crystal structureor a cubic crystal structure.

The front surface 11 of the single crystalline semiconductor-carbonalloy layer 10 is substantially planar, and may be provided by cutting,cleaving, grinding, and/or polishing. The front surface 11 of the singlecrystalline semiconductor-carbon alloy layer 10 is not orthogonal to anymajor crystallographic orientation of the single crystallinesemiconductor-carbon alloy layer 10. Specifically, the front surface 11of the single crystalline semiconductor-carbon alloy layer 10 isslightly miscut relative to a crystallographic orientation of the singlecrystalline semiconductor-carbon alloy layer 10. Typically, the frontsurface 11 of the single crystalline semiconductor-carbon alloy layer 10is slightly miscut relative to a major crystallographic orientation ofthe single crystalline semiconductor-carbon alloy layer 10. The angle ofthe miscut is between 0 degree and 3 degrees. Thus, the front surface 11of the single crystalline semiconductor-carbon alloy layer 10 is at anangle between 88 degrees and 90 degrees, and typically between 88degrees and 89.999 degrees, and more typically between 88 degrees and89.99 degrees, with a crystallographic orientation of the singlecrystalline semiconductor-carbon alloy layer 10. In one embodiment, thesurface normal of the front surface 11 of the single crystallinesemiconductor-carbon alloy layer 10 deviates from a majorcrystallographic orientation of the single crystallinesemiconductor-carbon alloy layer 10 by an angle not greater than 2degrees, and preferably by an angle not greater than 1 degree, and morepreferably by an angle not greater than 0.3 degrees.

Exemplary major crystallographic orientations of the single crystallinesemiconductor-carbon alloy layer 10 include, but are not limited to,<100> orientations, <110> orientations, <111> orientations, <210>orientations, <211> orientations, <221> orientations, <310>orientations, <311> orientations, <320> orientations, <321>orientations, and <322> orientations in case the single crystallinesemiconductor-carbon alloy layer 10 has a cubic crystal structure.Exemplary major crystallographic orientations of the single crystallinesemiconductor-carbon alloy layer 10 include, but are not limited to,<0001> orientations, <1102> orientations, <1000> orientations, and<1100> orientations in case the single crystalline semiconductor-carbonalloy layer 10 has a hexagonal crystal structure.

In one embodiment, the single crystalline semiconductor-carbon alloylayer 10 can be a commercially available single crystalline substrate ofa semiconductor-carbon alloy such as silicon carbide. Currently, siliconcarbide substrates having a diameter equal to or greater than 4 inchesare commercially available. Such a silicon carbide substrate may havescratches and surface roughness that are typical of commerciallyavailable silicon carbide substrates. Appropriate surface clearing suchas an ultrasonic clean in a cleaning solution may be performed to removeany impurities on the front surface 11 of the single crystallinesemiconductor-carbon alloy layer 10.

Referring to FIG. 3, semiconductor islands 20 are deposited on the frontsurface 11 of the single crystalline semiconductor-carbon alloy layer10. Each of the semiconductor islands 20 are deposited as a discretestructure that does not contact any other semiconductor island 20. Inother words, the semiconductor islands 20 do not contact one anotherimmediately after formation.

In one embodiment, the semiconductor islands 20 include the samesemiconductor material as a component semiconductor material of thesingle crystalline semiconductor-carbon alloy layer 10. The componentsemiconductor material refers to the non-carbon material of the singlecrystalline semiconductor-carbon alloy layer 10, which would have formeda semiconductor material but for the presence of the carbon atoms. Forexample, if the single crystalline semiconductor-carbon alloy layer 10is a silicon carbide layer, the semiconductor islands 20 may be siliconislands. If the single crystalline semiconductor-carbon alloy layer 10is a germanium carbide layer, the semiconductor islands 20 may begermanium islands. If the single crystalline semiconductor-carbon alloylayer 10 is a carbide layer of an alloy of silicon and germanium, thesemiconductor islands 20 may be islands of a silicon-germanium alloy. Ifthe single crystalline semiconductor-carbon alloy layer 10 is a carbidelayer of a compound semiconductor, the semiconductor islands 20 may beislands of the compound semiconductor. In case the single crystallinesemiconductor-carbon alloy layer 10 includes more than one componentsemiconductor material, the composition of the semiconductor islands 20may, or may not match, the composition of the semiconductor materials inthe single crystalline semiconductor-carbon alloy layer 10. For example,if the single crystalline semiconductor-carbon alloy layer 10 is acarbide layer of an alloy of silicon and germanium, the semiconductorislands 20 may be islands of a silicon-germanium alloy, silicon islands,or germanium islands.

The deposition of the semiconductor islands 20 can be effected in manydifferent ways. Optionally, the single crystalline semiconductor-carbonalloy layer 10 can be degassed at an elevated temperature to evaporateany volatile material (e.g., water vapor) from the front surface 10. Thedegassing may be performed at a temperature from 100° C. to 1,500° C. ina vacuum environment or in an inert ambient gas such as He, Ne, Ar, Kr,and/or Xe. For example, the degassing may be performed in an ultrahighvacuum, i.e., at a pressure less than 1.0×10⁻⁶ Torr, or in an ambient inwhich the partial pressure of an inert gas is from 1.0×10⁻⁶ Torr to 760Torr provided by backfilling a vacuum chamber with the inert gas.

After the optional degassing, the semiconductor islands 20 aredeposited. In one embodiment, the semiconductor islands 20 can bedeposited by providing a reactant gas to the surface of the singlecrystalline semiconductor-carbon alloy layer 10 in a process chamber,which can be a vacuum chamber. The reactant gas can be a compoundincluding the component semiconductor material. For example, if thesingle crystalline semiconductor-carbon alloy layer 10 is a singlecrystalline silicon carbide layer, the reactant gas can be asilicon-containing precursor gas such as Si₂H₆ (disilane), SiH₄(silane), SiH₂Cl₂ (dichlorosilane), SiHCl₃ (trichlorosilane), and SiCl₄(silicon tetrachloride). If the single crystalline semiconductor-carbonalloy layer 10 is a single crystalline germanium carbide layer, thereactant gas can be a silicon-containing precursor gas such as Ge₂H₆(digermane), GeH₄ (germane), GeH₂Cl₂ (germanium dichloride), and GeCl₄(germanium tetrachloride). The reactant gas may be delivered into theprocess chamber without a carrier gas, or with a carrier gas. Thecarrier gas may be hydrogen or an inert gas such as He, Ne, Ar, Kr,and/or Xe.

Formation of the semiconductor islands 20 as discrete structures, i.e.,structures that do not contact one another, can be effected bydepositing the semiconductor islands 20 at a temperature at whichsurface diffusion of the component semiconductor material is limited.Specifically, the deposition temperature for the semiconductor islands20 is selected such that the surface diffusion of the semiconductormaterial of the semiconductor islands 20 is sufficiently limited toprevent formation of a contiguous layer of the semiconductor material.

The selection of a deposition temperature in a range that provideslimited surface diffusion induces deposition of the semiconductormaterial originating from the reactant gas in Volmer-Weber growth mode.In the Volmer-Weber growth mode, the semiconductor adatoms on the frontsurface 11 of the single crystalline semiconductor-carbon alloy layer 10form three-dimensional adatom clusters of isolated islands, therebyforming the semiconductor islands 20 as discrete structures. In general,the lateral dimensions of the semiconductor islands can be from 10 nm to500 nm, and typically from 50 nm to 300 nm, although lesser and greaterlateral dimensions can also be formed depending on depositionconditions.

For example, if the single crystalline semiconductor-carbon alloy layer10 is a single crystalline silicon carbide layer and the reactant gas isdisilane, the deposition temperature for the semiconductor islands 20,which are silicon islands in this case, can be performed at atemperature from 500° C. to 850° C., and preferably from 550° C. to 800°C. The flow of the reactant gas, which is disilane in this case, dependson the size of the crystalline silicon carbide layer and the processchamber. For a single crystalline semiconductor-carbon alloy layer 10 inthe form of a four-inch diameter substrate, the disilane flow of 0.1sccm to 100 sccm may be employed depending on the temperature and thepresence or absence of a carrier gas. In this example, the depositiontime may be from 10 minutes to 12 hours to form silicon islands having amaximum height of about 200 nm.

In another embodiment, the semiconductor islands 20 are deposited byproviding a molecular beam of the component semiconductor material tothe front surface 11 of the single crystalline semiconductor-carbonalloy layer 10. The molecular beam may be generated from a heated sourcethat evaporates the component semiconductor material, e.g., silicon,germanium, or a compound semiconductor material. The heated source maybe an effusion cell, an electron-beam heated evaporation source, or anyother device configured to generate a continuous or periodic beam of thecompound semiconductor material. The molecular beam of the componentsemiconductor material may be directed to the front surface 11 in anultrahigh vacuum environment or in an ambient including hydrogen or aninert gas such as He, Ne, Ar, Kr, and/or Xe.

As in the case of deposition from a reactant gas, the depositiontemperature for formation of the semiconductor islands 20 from amolecular beam is selected such that the surface diffusion of thesemiconductor material of the semiconductor islands 20 is sufficientlylimited to prevent formation of a contiguous layer of the semiconductormaterial. The selection of a deposition temperature in a range thatprovides limited surface diffusion induces deposition of thesemiconductor material originating from the reactant gas in Volmer-Webergrowth mode. In general, the lateral dimensions of the semiconductorislands can be from 10 nm to 500 nm, and typically from 50 nm to 300 nm,although lesser and greater lateral dimensions can also be formeddepending on deposition conditions.

For example, if the single crystalline semiconductor-carbon alloy layer10 is a single crystalline silicon carbide layer and the molecular beamis a beam of silicon atoms, the deposition temperature for thesemiconductor islands 20, which are silicon islands in this case, can beperformed at a temperature from 500° C. to 850° C., and preferably from550° C. to 800° C. In this example, the deposition time may be from 10minutes to 12 hours to form silicon islands having a maximum height ofabout 200 nm.

Referring to FIGS. 4A and 4B, atomic force microscopy (AFM) images of anarea including silicon islands on a single crystalline silicon carbidesurface are shown. FIG. 4A is a 10 μm×10 μm image of the area generatedin the height mode, in which the brightness of the image is proportionalto the vertical displacement of the tip of the AFM probe. FIG. 4B is a10×10 μm image of the same area generated the amplitude mode, in whichthe brightness of the image is proportional to changes in the phase ofthe measured signal, and is related to the spatial derivative of theheight of the tip of the AFM probe.

Referring to FIG. 4C, another AFM image shows silicon islands formed ona single crystalline silicon carbide surface. FIG. 4C is an image of a 5μm×5 μm area.

Referring to FIG. 4D, yet another AFM image shows silicon islands formedon a single crystalline silicon carbide surface. FIG. 4D is an image ofa 1.5 μm×1.5 μm area.

The images of FIGS. 4A-4D show examples of silicon islands that areformed on the surface of a silicon carbide layer. While the images ofFIGS. 4A-4D show silicon islands having a lateral dimension of about 150nm, changes in the deposition conditions can alter the lateraldimensions of the silicon islands. The silicon islands do not contactone another.

Referring to FIG. 5, a substrate 30 having a second front surface 31 isplaced on the silicon islands 20 such that the second front surface 31contacts some of the semiconductor islands 20. The substrate 30 isplaced such that the second front surface 31 faces the front surface 11of the single crystalline semiconductor-carbon alloy layer 10.

The substrate 30 can be another single crystalline semiconductor-carbonalloy layer, which may, or may not, have the same composition as thesingle crystalline semiconductor-carbon alloy layer 10. If the substrate30 is another single crystalline semiconductor-carbon alloy layer, thesecond front surface 31 can be substantially planar. Like the frontsurface 11 of the single crystalline semiconductor-carbon alloy layer10, the second front surface 31 may be provided by cutting, cleaving,grinding, and/or polishing. Further, the second front surface 31 of theother single crystalline semiconductor-carbon alloy layer of thesubstrate 30 is not orthogonal to any major crystallographic orientationof the other single crystalline semiconductor-carbon alloy layer of thesubstrate 30. Specifically, the second front surface 31 of the othersingle crystalline semiconductor-carbon alloy layer of the substrate 30is slightly miscut relative to a crystallographic orientation of theother single crystalline semiconductor-carbon alloy layer of thesubstrate 30. Typically, the second front surface 31 of the other singlecrystalline semiconductor-carbon alloy layer of the substrate 30 isslightly miscut relative to a major crystallographic orientation of theother single crystalline semiconductor-carbon alloy layer of thesubstrate 30. The angle of the miscut is between 0 degree and 3 degrees.Thus, the second front surface 31 of the other single crystallinesemiconductor-carbon alloy layer of the substrate 30 is at an anglebetween 88 degrees and 90 degrees, and typically between 88 degrees and89.999 degrees, and more typically between 88 degrees and 89.99 degrees,with a crystallographic orientation of the other single crystallinesemiconductor-carbon alloy layer of the substrate 30. In one embodiment,the surface normal of the second front surface 31 of the other singlecrystalline semiconductor-carbon alloy layer of the substrate 30deviates from a major crystallographic orientation of the other singlecrystalline semiconductor-carbon alloy layer of the substrate 30 by anangle not greater than 2 degrees, and preferably by an angle not greaterthan 1 degree, and more preferably by an angle not greater than 0.3degrees.

Alternately, the substrate 30 may include a material having a highmelting temperature and a low vapor pressure such as a refractory metalor a dielectric material such as sapphire (aluminum oxide) or diamond.In this case, the second front surface 31 of the substrate 30 can be asurface of a refractory metal or a surface of a dielectric materialhaving a high melting point, i.e., a melting point higher than thetemperature of the anneal, and a low vapor pressure, i.e., a vaporpressure that does not affect the surface properties of the frontsurface 11 at a subsequent anneal step.

In one embodiment, the substrate 30 is another single crystallinesemiconductor-carbon alloy layer which has the same composition as thesingle crystalline semiconductor-carbon alloy layer 10. For example, thesingle crystalline semiconductor-carbon alloy layer 10 and the substrate30 can be single crystalline silicon carbide layers and thesemiconductor islands 20 can be silicon islands.

While FIG. 5 schematically represents semiconductor islands 20 thatcontact the second front surface 31 of the substrate 30, not everysemiconductor island 20 contacts the second front surface 31 because thefront surface 11 of the single crystalline semiconductor-carbon alloylayer 10 and the second front surface 31 of the substrate 30 may haveglobal topography such as a bow or local topography such as scratches orsteps derived from surface preparation.

The assembly of the single crystalline semiconductor-carbon alloy layer10, the semiconductor islands 20, and the substrate 30 is placed in aprocess chamber, which may be the same chamber employed for formation ofthe semiconductor islands 20 or may be a different chamber. The processchamber may be a vacuum enclosure or may be configured to maintain acontrolled ambient including an inert gas. The process chamber isprovided with a heating mechanism to enable annealing of the assembly ofthe single crystalline semiconductor-carbon alloy layer 10, thesemiconductor islands 20, and the substrate 30.

The anneal may be performed in an ultrahigh vacuum environment, i.e., ata pressure below 1.0×10⁻⁶ Torr, or may be performed in an inert ambientcontaining an inert gas such as He, Ne, Ar, Xe, and/or Kr. If an ambientgas is employed, the pressure of the ambient gas may be from 0.1 mTorrto 760 Torr, and typically from 1 mTorr to 100 mTorr.

The temperature of the anneal is selected so that the semiconductormaterial of the semiconductor islands 20 evaporates at a significantrate during the anneal. Specifically, the temperature of the anneal canbe selected to be above the melting temperature of the semiconductormaterial of the semiconductor islands 20 so that the semiconductormaterial evaporates during the anneal. For example, if the singlecrystalline semiconductor-carbon alloy layer 10 is a silicon carbidelayer and the semiconductor islands 20 are silicon islands, the annealcan be performed at a temperature greater than 1,414° C. If the singlecrystalline semiconductor-carbon alloy layer 10 is a germanium carbidelayer and the semiconductor islands 20 are germanium islands, the annealcan be performed at a temperature greater than 938.25° C. The annealtemperature for a single crystalline semiconductor-carbon alloy layer 10having a different composition can be determined based on thecomposition of the semiconductor islands 20. In general, the annealtemperature can be selected so that the entirety of the semiconductorislands 20 can be evaporated in a time period ranging from 10 seconds to1 hour, and preferably from 30 seconds to 10 minutes, although longerand shorter time periods can also be employed. In general, the annealingis performed at a temperature that induces evaporation of thesemiconductor islands 20 from between the front surface 11 of the singlecrystalline semiconductor-carbon alloy layer 10 and second front surface31 of the substrate 30.

Referring to FIG. 6, the structure of the assembly is shown during theanneal. The assembly includes the single crystallinesemiconductor-carbon alloy layer 10, the semiconductor islands 20, andthe substrate 30. The size of the semiconductor islands 20 shrink duringthe anneal as the semiconductor material evaporates from thesemiconductor islands 20 in the from of an atomic vapor. The spacebetween the front surface 11 of the single crystallinesemiconductor-carbon alloy layer 10 and the second front surface 31 ofthe substrate 30 maintains a high vapor pressure of the semiconductormaterial, which is evaporated from the semiconductor islands 20.

The range of the distance between the front surface 11 of the singlecrystalline semiconductor-carbon alloy layer 10 and the second frontsurface 31 of the substrate 30 is the greater of the combined surfaceroughness of the front surface 11 and the second front surface 31 andthe vertical dimension of the semiconductor islands 20. Typically, thecombined surface roughness of the front surface 11 and the second frontsurface 31 is on the order of several microns, and the verticaldimension of the semiconductor islands 20 is less than 1 micron. Thelateral dimensions of the single crystalline semiconductor-carbon alloylayer 10 and the substrate 30 are on the order of several inches. Thus,the geometry of the assembly of the single crystallinesemiconductor-carbon alloy layer 10, the semiconductor islands 20, andthe substrate 30 is conducive to maintaining a high vapor pressure ofthe semiconductor material of the semiconductor islands 20 throughoutthe space between the front surface 11 and the second front surface 31throughout the anneal.

The high vapor pressure of the semiconductor material of thesemiconductor islands 20 in the space between the front surface 11 andthe second front surface 31 provides a sufficient influx ofsemiconductor atoms to the front surface 11 and the second front surface31. Because of the constant influx of the semiconductor atoms,semiconductor atoms selectively evaporated from the front surface 31 ofthe single crystalline semiconductor-carbon alloy layer 10 is constantlyreplenished by other semiconductor atoms from the semiconductor islands20. Selective evaporation of the semiconductor atoms refers to loss ofsemiconductor atoms while carbon atoms do not evaporate proportionallyfrom the surface of a semiconductor-carbon alloy. Thus, the high vaporpressure of the semiconductor material of the semiconductor islands 20has the effect of retarding the selective loss of semiconductor atomsfrom the single crystalline semiconductor-carbon alloy layer 10 andpreventing initiation of graphitization, i.e., formation of a layerincluding only carbons on the surface of a semiconductor-carbon alloy.

While the graphitization is postponed, the high temperature of theanneal process provides sufficient mobility to the semiconductor atomsand the carbon atoms on the front surface 11 of the single crystallinesemiconductor-carbon alloy layer 10. This results in rearrangement ofmicroscopic defects on the front surface 11 of the single crystallinesemiconductor-carbon alloy layer 10. The microscopic defects are healedduring the anneal. The microscopic surfaces, or ledges that areperpendicular to a major crystallographic orientation, are formed andgrows. The major crystallographic orientation slightly deviates from themacroscopic orientation of the front surface 11. The process offormation of ledges on a microscopic scale results in conversion of thefront surface 11 into a vicinal surface 13 during the annealing.

If the substrate 30 is another single crystalline semiconductor-carbonalloy layer, the high vapor pressure of the semiconductor material ofthe semiconductor islands 20 has the same effect of retarding theselective loss of semiconductor atoms from the other single crystallinesemiconductor-carbon alloy layer of the substrate 30 and preventinginitiation of graphitization on the second front surface 31. While thegraphitization is postponed on the second front surface 31, the hightemperature of the anneal process provides sufficient mobility to thesemiconductor atoms and the carbon atoms on the second front surface 31of the other single crystalline semiconductor-carbon alloy layer of thesubstrate 30. This results in rearrangement of microscopic defects onthe second front surface 31 of the other single crystallinesemiconductor-carbon alloy layer of the substrate 30, in which themicroscopic defects are healed and microscopic surfaces, or ledges, thatare perpendicular to a major crystallographic orientation that slightlydeviates from the macroscopic orientation of the second front surface31. While the graphitization is postponed, the high temperature of theanneal process provides sufficient mobility to the semiconductor atomsand the carbon atoms on the second front surface 31 of the singlecrystalline semiconductor-carbon alloy layer of the substrate 30. Thesame mechanism that operates on the front surface 11 causes theconversion of the second front surface 31 into a second vicinal surface33 during the annealing.

For example, if the single crystalline semiconductor-carbon alloy layer10 and the substrate 30 can be single crystalline silicon carbidelayers, and the vicinal surface 13 and the second vicinal surface 33 canbe vicinal surfaces of silicon carbide layers.

Referring to FIG. 7, a schematic vertical cross-sectional view of asubstrate illustrates the concept of a vicinal surface formed accordingto an embodiment of the present disclosure. Each circle represents aunit of a crystal structure, which may be a unit cell. The vicinalsurface includes ledges separated by steps, which has the height of asingle atomic bilayer. The vicinal surface may be viewed as a collectionof periodically arranged or near-periodically arranged ledges having amajor crystallographic orientation in which each step has the height ofa single atomic layer. The width of each ledge may be regular, or may beirregular with a statistical distribution of widths. The vicinal surfacecan have at least 10 atoms along a widthwise direction of a single ledge(between two adjacent steps along an almost horizontal direction in FIG.7), and typically has at least 30 atoms along the widthwise direction ofa single ledge, and more typically has about 100 atoms or more along thewidthwise direction of a single ledge.

Correspondingly, the angle between the surface normal and the majorcrystallographic orientation is relatively small, and, for example, isfrom 0 degree to 2 degrees, and typically is from 0 degree to 1 degree.Because a vicinal surface is not along any major crystallographicorientation, at least one of the Miller indices is a significantly largenumber. For example, at least one of the Miller indices representing thevicinal surface is greater than 6, and is typically greater than 30.

Vicinal surfaces are known to provide benefits for certain applications.For example, a vicinal surface of silicon carbide may be able to providea high quality graphene layer. A graphene monolayer has a thickness ofabout 0.34 nm, i.e., which is approximately the atomic diameter of asingle carbon atom.

Referring to FIG. 8, the evaporation of the semiconductor islands 20continues during the anneal until the semiconductor islands 20 disappearfrom the assembly of the single crystalline semiconductor-carbon alloylayer 10, the semiconductor islands 20, and the substrate 30. At thispoint, the assembly includes only the single crystallinesemiconductor-carbon alloy layer 10 and the substrate 30. The singlecrystalline semiconductor-carbon alloy layer 10 has the vicinal surface13. The substrate 30 may have the second vicinal surface 33 if thesubstrate 30 includes the other single crystalline semiconductor-carbonalloy layer. Local gaps (not shown) may be present between the vicinalsurface 13 of the single crystalline semiconductor-carbon alloy layer 10and the substrate 30 due to the macroscopic and microscopic topographyof the vicinal surface 13 of the single crystalline semiconductor-carbonalloy layer 10 and the second vicinal surface 33 (or the second frontsurface 31 if the substrate 30 does not include another singlecrystalline semiconductor-carbon alloy layer).

Referring to FIG. 9, the anneal may be stopped immediately before anysignificant graphitization takes place, e.g., immediately afterconsumption of all semiconductor islands 20, according to a firstembodiment of the present disclosure. The single crystallinesemiconductor-carbon alloy layer 10 and the substrate 30 may be cooledto room temperature, taken out of the process chamber, and be separatedfrom each other to provide the single crystalline semiconductor-carbonalloy layer 10 having the vicinal surface 13. If the substrate 30includes another single crystalline semiconductor-carbon alloy layer, asecond vicinal surface 33 may be provided on a surface of the substrate30.

Referring to FIG. 10, the anneal may be continued at the sametemperature or at a different temperature to induce graphitization onthe vicinal surface 13 of the single crystalline semiconductor-carbonalloy layer 10 according to a second embodiment of the presentdisclosure. Once the entirety of the semiconductor islands is consumed,the vapor pressure of the semiconductor material in the space betweenthe single crystalline semiconductor-carbon alloy layer 10 and thesubstrate 30 are drastically reduced. As a result, a surface layerincluding an excess of carbon atoms relatively to the composition of thesemiconductor-carbon alloy is formed on the surface of the singlecrystalline semiconductor-carbon alloy layer 10. A graphene layer 15including at least one graphene monolayer can be formed at the vicinalsurface as semiconductor atoms evaporate from the vicinal surface 13.The semiconductor atoms that evaporate from the vicinal surface 13reduce the semiconductor content of the vicinal surface, and theremaining carbon atoms coalesce to form the graphene layer 15. In oneembodiment, the graphene layer 15 may have an epitaxial alignment withthe underlying crystalline structure of the single crystallinesemiconductor-carbon alloy layer 10.

The graphene layer 15 can exist as a monolayer of a two-dimensionalsheet. Alternately, the graphene layer 15 can exist as a stack of aplurality of two-dimensional monolayers of carbon, which do not exceedmore than 10 monolayers and is typically limited to less than 5monolayers. Graphene provides excellent in-plane conductivity. In thegraphene layer 15, carbon atoms are arranged in a two-dimensionalhoneycomb crystal lattice in which each carbon-carbon bond has a lengthof about 0.142 nm. The graphene layer 15 on the vicinal surface 13 ofthe single crystalline semiconductor-carbon alloy layer 10 can be a highquality graphene layer having sufficient width to providetwo-dimensional properties to all areas of the graphene layer 15. Assuch, semiconductor devices employing the graphene layer 15 can befabricated on the single crystalline semiconductor-carbon alloy layer 10to provide high-density, high-switching-speed semiconductor circuits.

Each monolayer within the graphene layer 15 formed on the vicinalsurface 13 of the single crystalline semiconductor-carbon alloy layer 10typically has a substantial width, which corresponds to the width of aledge of the vicinal surface 13. As such, each monolayer of the graphenelayer 15 has a minimal width at each location, which provides thegraphene layer 15 a two-dimensional characteristic over a wide area. Byforming a high quality vicinal surface 13 that is more periodic and lessdefective than original surfaces provided by mechanical means alone, thequality of the graphene layer 15 according to this embodiment of thepresent disclosure is enhanced. Because the graphene layer 15 followsthe contour of the underlying vicinal surface 13, the outer surface ofthe graphene layer 15 is also a vicinal surface, i.e., a vicinal surfaceof the graphene layer 15.

In case the substrate 30 includes another single crystallinesemiconductor-carbon alloy layer, a second graphene layer 35 can beformed on the second vicinal surface 33 of the other crystallinesemiconductor-carbon alloy layer in the substrate 30. The secondgraphene layer 35 includes at least one graphene monolayer like thegraphene layer 15 on the single crystalline semiconductor-carbon alloylayer 10. In case each of the single crystalline semiconductor-carbonalloy layer 10 and the substrate 30 includes a single crystallinesilicon carbide layer, the graphene layer 15 and the second graphenelayer 35 are formed on the vicinal surface 13 and the second vicinalsurface 33, respectively. In this case, the vicinal surface 13 and thesecond vicinal surface 35 can include ledges having a width greater than100 nm and steps having a height of a single monolayer of siliconcarbide between adjacent ledges.

Referring to FIG. 11, The single crystalline semiconductor-carbon alloylayer 10 and the substrate 30 may be cooled to room temperature, takenout of the process chamber, and be separated from each other to providethe single crystalline semiconductor-carbon alloy layer 10 having thevicinal surface 13.

Referring to FIGS. 12A, 12B, and 12C, atomic force microscopy (AFM)images of vicinal surfaces including a graphene layer are shown. Thegraphene layer was formed on a silicon carbide substrate sample having adiameter of about four inches. Silicon islands were deposited on thesilicon carbide substrate sample, and another silicon carbide substratewas placed on the silicon islands according to the methods of thepresent disclosure as described above. The assembly of the siliconcarbide substrate sample, the silicon islands, and the other siliconcarbide substrate was annealed at 1,450° C. for about two minutes. Afterformation of the graphene layer on the silicon carbide substrate sample,the AFM images were taken from the silicon carbide substrate sample.

FIG. 12A is 40 μm×40 μm image of a center area of the silicon carbidesubstrate sample as generated in the height mode, in which thebrightness of the image is proportional to the vertical displacement ofthe tip of the AFM probe. FIG. 12B is a 5 μm×5 μm image of anothercenter area of the silicon carbide substrate sample generated the heightmode. FIG. 12C is a 20×20 μm image of an edge area of the siliconcarbide substrate sample generated the height mode.

The lines in each of the AFM images of FIGS. 12A-12C correspond toatomic steps between adjacent ledges in a graphene layer, whichcorresponds to underlying atomic steps in the silicon carbide layer. Theimages of FIGS. 12A-12C show that the center area of the silicon carbidesubstrate sample, which was subjected to a higher vapor pressure ofsilicon during the anneal than the edge areas of the silicon carbidesubstrate sample, provides a better uniformity in the distribution ofthe widths of ledges. However, even the edge area of the silicon carbidesubstrate sample, as evidenced by the AFM image of FIG. 12C, showssignificantly wide ledges having a lateral dimension on the order of onemicron. When compared with the AFM image of FIG. 1 that does not showany sign of vicinal surfaces on the surface, the AFM images of FIGS.12A-12C show that vicinal surfaces are present in the silicon carbidesubstrate sample, and that the presence of the vicinal surface has abeneficial effect of providing a high quality graphene layer.

Referring to FIG. 13, the Raman spectroscopy data from a surface of thesilicon carbide substrate sample employed to generate the AFM images ofFIGS. 12A-12C. The horizontal axis shows the wavenumber, and thevertical axis shows the count in an arbitrary unit. The Raman spectrumshows the presence of a G-peak around 1,600 cm⁻¹ and a 2D peak around2,750 cm⁻¹. Further, there is a conspicuous absence of a D-peak at 1,350cm⁻¹ that is attributed to defects. The combination of the presence ofthe G-peak and the 2D peak and the absence of the D-peak shows that thegraphene layer of the silicon carbide substrate sample is a high qualitygraphene layer free of defects. The pattern of the Raman spectrumsuggests that the graphene layer on the silicon carbide substrate sampleis one monolayer thick.

Various devices can be formed employing a graphene layer 15 on a singlecrystalline semiconductor-carbon alloy layer 10 formed by the methodsdescribed above. Such devices can include a graphene-based field effecttransistor, i.e., a “graphene FET,” or any other electronic devicesemploying the electronic transport properties of the graphene layer 15.

Referring to FIG. 14, an exemplary graphene-based field effecttransistor (graphene FET) according to an embodiment of the presentdisclosure can be formed employing a graphene layer 15 on a singlecrystalline semiconductor-carbon alloy layer 10 as an exemplaryapplication. The structure of a graphene FET and the method ofmanufacturing the same described herein are non-limiting embodiments,and any other compatible structure or method for a graphene FET can alsobe employed.

Lithography techniques can be employed to define the dimensions of agraphene based device to be formed. The lithographic patterning of thegraphene layer 15 can be effected by masking the desired area of thegraphene layer 15 with a photoresist (photoresist), which can be, forexample, a layer of poly(methyl methacrylate), i.e., PMMA. Thephotoresist is lithographically patterned by exposure and developmentinto a desired pattern, which can be, for example, a rectangular patternsuch that the width of the patterned photoresist is the desired widthfor the channel of a graphene based transistor to be subsequentlyformed. Employing the photoresist as an etch mask, the exposed portionsof the graphene layer 15 can be etched, for example, by subjecting tooxygen plasma the unmasked portions of the graphene layer 15. Thephotoresist 27 is then removed, for example, by dissolving in a solvent.

A gate dielectric 140 and a gate electrode 150 are formed on a portionof the graphene layer 15. In one embodiment, the gate dielectric 140 caninclude a dielectric metal oxide, which can be deposited employingmethods known in the art including, but not limited to, chemical vapordeposition (CVD), atomic layer deposition (ALD), or a combination ofthereof. The dielectric metal oxide may include a high-k dielectricmaterial having a dielectric constant greater than 4.0, or any suitablecombination of these materials. Exemplary high-k dielectric materialsinclude dielectric metal oxides and dielectric metal oxynitrides such asHfO₂, ZrO₂, La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y),ZrO_(x)N_(y), La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y),SrTiO_(x)N_(y), LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and analloy thereof. Each value of x is independently from about 0.5 to about3 and each value of y is independently from 0 to about 2. Alternately orin addition, the gate dielectric 140 can include silicon oxide, siliconnitride, silicon oxynitride, or a combination thereof.

The gate electrode 150 includes a conductive material, such as ametallic material and/or a doped semiconductor material. A dielectricmaterial layer can be deposited in a conformal deposition step andanisotropically etched to form a dielectric gate spacer 160 thatlaterally surround the stack of the gate dielectric 140 and the gateelectrode 150.

Referring to FIGS. 8 and 8A, contact metal portions are deposited on theexemplary structure by method known in the art and lithographicallypattered to form a gate electrode 70, a source electrode 72, and a drainelectrode 74. Typically, the gate electrode 70, the source electrode 72,and the drain electrode 74 include a conductive material, which istypically a metal.

A contact level dielectric material layer 80 and conductive contact viastructures can be formed. The contact level dielectric material layer 80can be a homogeneous dielectric material layer or can be a stack of aplurality of different dielectric material layers. The conductivecontact via structures can be formed by etching via cavities in thecontact level dielectric material layer 80 and filling the via cavitieswith a conductive material, followed by removal of the excess conductivematerial from above the top surface of the contact level dielectricmaterial layer 80. The conductive contact via structures include agate-side contact via structure 90 contacting the gate electrode 70, asource-side contact via structure 92 contacting the source electrode 72,and a drain-side contact via structure 94 contacting the drain electrode74. The gate-side contact via structure 90, the source-side contact viastructure 92, and the drain-side contact via structure 94 are embeddedin the contact level dielectric material layer 80.

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Accordingly, the disclosure is intended toencompass all such alternatives, modifications and variations which fallwithin the scope and spirit of the disclosure and the following claims.

What is claimed is:
 1. A method of forming a structure, said methodcomprising: depositing semiconductor islands on a surface of a singlecrystalline semiconductor-carbon alloy layer by adding a semiconductormaterial onto said surface of said single crystallinesemiconductor-carbon alloy layer, said semiconductor islands having adifferent composition than said single crystalline semiconductor-carbonalloy layer and including a non-carbon material of said singlecrystalline semiconductor-carbon alloy layer and not including carbon;and annealing said single crystalline semiconductor-carbon alloy layerwhile a substrate is placed on said semiconductor islands, wherein saidsurface of said single crystalline semiconductor-carbon alloy layerbecomes a vicinal surface during said annealing.
 2. The method of claim1, wherein said semiconductor islands comprise a same semiconductormaterial as a component semiconductor material of said singlecrystalline semiconductor-carbon alloy layer.
 3. The method of claim 2,wherein said semiconductor islands are deposited by providing a reactantgas that is a compound including said component semiconductor materialto said surface of said single crystalline semiconductor-carbon alloylayer.
 4. The method of claim 2, wherein said semiconductor islands aredeposited by providing a molecular beam of said component semiconductormaterial to said surface of said single crystalline semiconductor-carbonalloy layer.
 5. The method of claim 2, wherein said semiconductorislands are deposited at a temperature at which surface diffusion ofsaid component semiconductor material is sufficiently limited to preventformation of a contiguous layer of said component semiconductormaterial.
 6. The method of claim 1, wherein said semiconductor islandsdo not contact one another immediately after formation.
 7. The method ofclaim 1, wherein said annealing is performed at a temperature thatinduces evaporation of said semiconductor islands from between saidsurface of a single crystalline semiconductor-carbon alloy layer andsaid substrate.
 8. The method of claim 1, wherein said surface of saidsingle crystalline semiconductor-carbon alloy layer is at an anglebetween 88 degrees and 89.999 degrees with a crystallographicorientation of said single crystalline semiconductor-carbon alloy layer.9. The method of claim 1, wherein said single crystallinesemiconductor-carbon alloy layer is a single crystalline silicon carbidelayer.
 10. The method of claim 9, wherein said semiconductor island aresilicon islands.
 11. The method of claim 1, wherein said substrateincludes another single crystalline semiconductor-carbon alloy layer,wherein a surface of said another single crystallinesemiconductor-carbon alloy layer faces said surface of said singlecrystalline semiconductor-carbon alloy layer during said annealing. 12.The method of claim 11, wherein said surface of said another singlecrystalline semiconductor-carbon alloy layer becomes a vicinal surfaceduring said annealing.
 13. The method of claim 1, wherein a graphenelayer including at least one graphene monolayer is formed at saidvicinal surface as semiconductor atoms evaporate from said vicinalsurface during said annealing.
 14. The method of claim 13, wherein saidsemiconductor islands comprise a same semiconductor material as acomponent semiconductor material of said single crystallinesemiconductor-carbon alloy layer.
 15. The method of claim 13, whereinsaid annealing is performed at a temperature that induces evaporation ofsaid semiconductor islands from between said surface of a singlecrystalline semiconductor-carbon alloy layer and said substrate.
 16. Themethod of claim 13, wherein said surface of said single crystallinesemiconductor-carbon alloy layer is is at an angle between 88 degreesand 89.999 degrees with a crystallographic orientation of said singlecrystalline semiconductor-carbon alloy layer.
 17. The method of claim13, wherein said single crystalline semiconductor-carbon alloy layer isa single crystalline silicon carbide layer.
 18. The method of claim 13,wherein said substrate includes another single crystallinesemiconductor-carbon alloy layer, wherein a surface of said anothersingle crystalline semiconductor-carbon alloy layer faces said surfaceof said single crystalline semiconductor-carbon alloy layer during saidannealing, and another graphene layer including at least one graphenemonolayer is formed at said of said another single crystallinesemiconductor-carbon alloy layer after said annealing.